Method and media for printing aqueous ink jet inks on plastic surfaces

ABSTRACT

Single coatings onto non-absorbent substrates on which aqueous ink jet inks are jetted with subsequent application of heat, without the use of reactive species in the ink and in the media used as a substrate coating. The heating rapidly drives the water of the ink into the coating, with the rest of the water from the ink being lost by evaporation. The material is then subject to immediate application of a sealing lacquer, which is preferably water-based and which may be dried and possibly cross-linked by heat to give a very resistant surface.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims priority from and is related to U.S.Provisional Patent Application Ser. No. 60/458,968, filed 1 Apr., 2003,this U.S. Provisional Patent Application incorporated by reference inits entirety herein.

FIELD OF INVENTION

The present invention relates to the production of robust plasticprinted products using aqueous inkjet inks for the imaging process.

BACKGROUND FOR THE INVENTION

Plastic objects now play a major part in modern life. Such objects aremarked on their surface for information and decoration. Often theplastic is handled or subject to forces of abrasion or other types ofwear, which could adversely affect any surface markings. Such damagecould reduce the effectiveness of any such markings.

Ink jet is a modern method of marking, which has had a large degree ofsuccess in the market for a variety of reasons. It is a non-impactprinting process, whereby ink is squirted through very fine nozzles andthe resultant ink droplets form an image directly on a substrate. Oneadvantage of the process is that the equipment can be made and soldrelatively inexpensively and for this reason it has been adapted forsale in the home as the prime method of printing from PC's (PersonalComputers). Another advantage is that because it is a non-impactprocess, printing can be done on fragile surfaces such as those of eggs.As the process does not work through a master, it can be used to printvariable information, where each print is different from the previousone. Working through a master means that once the master is imaged, theimage is fixed for as many impressions as required. After making therequired number of impressions, the master is discarded and a new onemade for the next job.

There are two main types of ink jet process. In one process, usuallytermed continuous ink-jet printing (CIJ), a stream of ink drops areelectrically charged and then are deflected by an electric field, eitherdirectly or indirectly, onto the substrate. In the second process,usually called Drop on Demand (DOD) ink-jet printing, the ink supply isregulated by an actuator such as a piezoelectric actuator. The pressureproduced during the actuation forces a droplet through a nozzle onto thesubstrate. CIJ inks need to have some electrical conductivity, but inksfor DOD ink-jet printing do not need to be conductive.

In all of the ink jet processes, in order to form suitable droplets, theink must have a relatively low viscosity during the actual jettingprocess. This is generally in the region of 1 to 30 centipoise. This canbe achieved by using a low viscosity carrier fluid, which may be water,or volatile organic liquid, or a relatively non-volatile organic liquid.Alternatively, the ink can be heated up to lower its viscosity duringjetting. For many reasons, water has been found to be a suitable carrierliquid. It is inexpensive, readily available, environmentally harmlessand has a high surface tension, which enables additives needed for theinkjet process to be used, as it is easier to find useful additives toreduce surface tension than to increase it.

Ink jet writing on plastic presents specific problems that inventorshave tried to solve. One such approach is to use inks other than aqueousbased, which can be made to dry and bond to plastic. WO 97/27053 byJennel et al describes the use of inkjet to digitally write on packagingmaterial. The printing can be done directly onto a pre-formed bottlesuch as one made from PET (polyester), or onto a carton blank or a webof packaging material. The invention is claimed to provide an advancedlevel of automation with minimum operator intervention. In order toachieve good adhesion to materials such as PET, ultra-violet (UV)sensitive inks are used and after jetting they are cured by UVradiation. The ink jet head is DOD and described as one supplied by thecompany Spectra. This is the most widely accepted way of using UV curinginkjet inks, as the alternative method, CIJ, generally uses water basedinks and the inks must contain electrically conductive material. UVsensitive inks are generally based on organic acrylate mixtures that donot contain electrically conductive ingredients and are therefore lesseasily adapted for use in CIJ.

UV sensitive inkjet inks are more expensive than water-based inks andwill remain so because, by definition, water-based inks contain a largequantity of water, which is relatively inexpensive. Because with UVinkjet inks all of the jetted material remains on the substrate surface(where the substrate is impermeable) inks are deposited in the form oftiny hemispherical structures. Process color work, where three or fourseparate inks are applied over the same area, can thus have aBraille-like feeling. Such an effect limits print quality.

The use of water-based inks on plastics would be advantageous forseveral reasons. As has been mentioned above, they have cost advantages;they can be used in both DOD and CIJ inkjet systems and they do notpile-up because the major part of the inkjet drop is water, whichdisappears either by absorption, if the substrate is pervious, or byevaporation, or both. However, there are a number of problems with usingwater-based inkjet inks on plastics. They have wetting problems withrelatively low energy plastic surfaces (for instance that of PET) aswell as slow drying, which for non-absorbent plastic surfaces has tooccur only by evaporation. Also, they have low wet smear resistance—i.e.after they have dried, they can be easily smeared with a wet finger.These problems have been appreciated for some years and for instanceU.S. Pat. No. 3,889,270 seeks to address this problem by using forinstance polyvinyl alcohol as a coating for the substrate and includingfor instance hydrophilic silica gel. U.S. Pat. No. 4,269,891 uses forinstance polyvinyl alcohol and pigments such as titanium dioxide forsuitable substrate coatings. U.S. Pat. No. 4,474,850 uses for instancesalts of high molecular weight carboxylic acids and locks the aqueousdye based inks into the surface by ionic interaction. U.S. Pat. No.4,474,850 is concerned with the production of transparencies. U.S. Pat.No. 4,592,951 expresses the need to cross-link the polyvinyl alcoholbecause uncross-linked layers are generally too tacky. Jones, in U.S.Pat. No. 4,649,064 employs hydrophilic film, partially cross-linked,onto which a cross-linkable ink jet ink is jetted. The ink dryingprocess is at room temperature. In general, in order to avoid the needfor customers to have special drying units, drying is expected to occurwithout any additional prompting with energy such as heat and thus it isat room temperature. The ink is then cross-linked with an agent withinthe substrate coating. WO 99/21724 by Wang et als. addresses the problemof ink smearing. The patent application describes the use of twolayers—an inner non-cross-linked hydrophilic coating and an outercross-linked hydrophilic coating. In one embodiment, an inkjet image isapplied before curing to avoid wet smear. Similarly, US 2001/0036552 byOtani et al. describes coating a substrate with two layers forwater-based pigment inks to give better colors and image fastness.

U.S. Pat. No. 5,537,137 addresses the problems of aqueous inkjetprinting on plastics by introducing a reactive material into thesubstrate coating and heating after depositing a reactive inkjet inkonto the coated substrate. The reaction fixes the ink into the coatingby the reaction of the ink with the coating and also cross-links thecoating itself to increase the coating durability. The disadvantages ofthis approach are that the presence of a reactive system, which curesthe background areas, limits shelf life of any such coated productbefore imaging, as the cross-linking reactions that occurs rapidly athigh temperatures also proceed slowly at room temperature. Also, inksused are limited to those containing reactive species either as a dye orpigment dispersion or as a polymeric material which serves as thepigment dispersant. If the energy used for fixing the ink is ultravioletlight, then the media must be protected from such light during handling.This may necessitate yellow light where the printing process occurs. Theresultant prints of such a process will show variable gloss on thesurface, reflecting the affect of the variations in absorption of water,dependent on the amount of ink deposited in the variable areas of theprinted material. An alternative approach for achieving smear-proofimages is to overcoat the ink jet image. U.S. Pat. No. 6,095,050describes the use of organic lacquers to fix aqueous ink jet inks intouncoated substrates where the ink jet ink is still wet during theovercoating process. This method works with paper where ink absorptioninto the substrate enables smudging of wet on wet to be avoided.

Much of the work for aqueous ink jet printing on plastic has beenaddressed to achieving smear-proof properties for the purposes ofhandling, for applications such as the imaging of overheadtransparencies. The present invention addresses more stringentrequirements for handling, where the printed object may be subject tohandling and solvents, which would damage ink jet images which weremerely smear proof. The method and materials below described for workingthe present invention provide the means of producing information ordecoration onto plastics using aqueous ink jet inks, resulting productsbeing of excellent image quality as well as being smear proof andabrasion resistant and generally having a high resistance to physicaland chemical damage, appropriate to the present applications.Applications for such methods and materials include cards used for ID'sor for smart card applications, where the product must undergo handlingand the unprinted material should benefit from a long shelf life. Afurther application is in the production of printing on bottles, wherebywhite substrate coating onto plastics such as PET (polyester) can beused to provide areas on which aqueous ink jet inks can be jettedfollowed by a protective overcoat, as well as providing the contrastingbackground for the transparent ink jet inks.

SUMMARY OF INVENTION

The present invention describes single coatings onto non-absorbentsubstrates on which aqueous ink jet inks are jetted with subsequentapplication of heat, without the use of reactive species in the ink andin the media used as a substrate coating. The heating rapidly drives thewater of the ink into the coating, with the rest of the water from theink being lost by evaporation. The image can thus be deemed to be dried.It has been found that even though rubbing with water may causesmearing, the media is sufficiently smudge proof to permit an immediateapplication of a sealing lacquer, which is preferably water-based andwhich may be dried and possibly cross-linked by heat to give a veryresistant surface. This surface may be made glossy or matt as required.Alternatively, the image can be made as a lateral inversion of therequired picture on a transparent substrate and then laminated to awhite substrate coated with a PSA (pressure sensitive adhesive); or byprinting on a white substrate or substrate coating and laminating with atransparent film.

In one aspect of the present invention, substrate coatings are describedthat are primarily designed for use in automated packaging. Suchcoatings are single layers and are hydrophilic. The layer itself mayhave very poor water resistance and may be removable with a damp rag.The lacquer overcoat produces an even coating on both the printed andunprinted areas, which may be either glossy or matt and which then makesthe resulting plastic water and smear resistant, solvent resistant andscratch resistant. In order to print process colors onto a plastic base,either the substrate itself may be opaque white, or the substratecoating may be colored with a white pigment such as titanium dioxide.The process colors of ink jet inks have a transparency, which gives anappearance similar to printing inks onto white paper.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of this invention, the layer coated directly onto thesubstrate and onto which the ink jet inks are jetted will be referred toas the under-coating and the mixture from which it is deposited will bereferred to as the under-coating solution, even though this solution maycontain dispersed particles or emulsions. In a preferred embodiment, theapplication as designed for automation does not require the initialhydrophilic under-coating to be handled and therefore it may be of afragile nature. As this is not the case with inkjet substrateunder-coats as designed for non-automated systems, there is latitude informulating such under-coats, which in the case of the present inventionis wider than usual. Generally, substrate coatings are subject tohandling—whether during manufacturing, packaging of the coatings or inthe actual imaging process. For instance, they may require resistance toabsorption of moisture from fingers and this would therefore restrictthe content of hydrophilic constituents in the coating. They should notbe physically weak and this limits the presence of excess pigment orfiller to give good layer opacity or fast ink absorption into thecoating. In the present application such coatings, whilst havingcharacteristics of a solid film, may have poor adhesion to the substrateand poor water resistance and may be easily damaged if the surfacecontacts another surface or is handled in any way, until after imagingand lacquering, when excellent physical and chemical adhesion can beachieved. The coatings may be applied to a wide range of substrates butare particularly suitable for plastics such as polyester (PET) andpolyvinyl chloride (PVC). The coating may be pigmented or transparent,depending on the application. A substrate with a white pigment, eitherincorporated therein or incorporated in the substrate under-coating, haswide application as it provides an essential background for transparentprocess inks. In the case of a coating, it can be applied to adesignated area of the substrate and the inkjet inking can be used injust this area. This is particularly useful for bottles of drink wherethe color of the drink can be seen through the transparent parts of thebottle and the bottle can still have an attractive aqueous inkjet imageaffixed to a white area provided by the substrate under-coating, so thatthe color fidelity can be maintained.

It is preferable but not essential to deposit the substrateunder-coating from aqueous solution.

It is also preferable, depending on the application, that the substrateunder-coating materials are chosen from only those approved for fooditems, making their use in the food and drink industry applicable.

The method of application for use on an automated production line is asfollows: apply the solution of substrate under-coating to the plasticsurface; air dry to evaporate the water or solvent; apply the aqueousinkjet inks in the form of the required image; treat with heat oranother form of energy to sufficiently dry the inkjet image into thesubstrate; then overcoat with lacquer and dry the lacquer—either to cureit, if it is cross-linkable or to drive off the water, if it is anemulsion. The overcoat then forms a protective insoluble film over boththe background and the image. The entire surface of the printedsubstrate may now be scratch proof and proof against damage fromsolvents, oils and water. This method will be referred to below as the“automated” method.

In the less automated form, the substrate under-coating and initialdrying may be done as a manufacturing process for supplying the coatedsubstrate to a customer. The customer will then image the coatedsubstrate with the aqueous inkjet inks and either reverse laminate orcoat with a lacquer—preferably aqueous—to produce the finished item.This method will be referred to below as the less automated method.

Both above described methods can be applied not only to bottles andother packaging items, but also to other ink jet applications such asprinting on credit cards, smart cards etc.

It is part of the invention that the color quality of the inkjet imageis retained or achieved after the applied energy finishing stage hasbeen completed to finalize the cross-linking process. Aqueous ink jetinks may be based on dye colorants or pigments and may containtechnologies to enhance drying and wet strength. Thus, although withsome aqueous ink formulations it may be possible to insolubilize them onuncoated plastic substrate, because of surface energy considerations,image quality may be lost completely as the inks often reticulate onplastic surfaces. Thus the under-coatings on the plastic substrate areessential for good ink jet printing.

Suitable formulations for applying to the substrate may be water-basedmixtures of polyvinyl alcohol and polyacrylic acid, together with awater-based emulsion containing a hydrophobic polymer in the internalphase and stabilized at a pH of 7 or less. Examples of suitablehydrophobic water-based emulsions are acrylic type materials such asFlexobond 325 (a vinyl-acrylic copolymer—pH of 4 to 5), Walpol 40-136 avinyl-acrylic copolymer—pH 5.0, and Flexobond 381 (a vinyl acrylic pH4-6).

In all of the above applications, the mixture can also contain titaniumdioxide or a mixture of white or opaqueing pigments dispersed therein,and fillers such as silicas and clays as are well known in the art.

Mixtures of use in this invention, when deposited on a plasticsubstrate, can be dried with warm air and give solid films. Depositionmay be by spraying or by any other suitable means of coating.

Suitable formulations may also be organic solvent-based mixtures of ananalogous composition to the water-based materials. Hydrophiliccellulose derivatives such as hydroxypropyl cellulose, which is solublein both water and organic solvent and plays a role similar to thepolyvinyl alcohol in the water-based compositions, may be deposited fromsolvents such as alcohol/ethyl acetate mixtures. The formulation maystill contain polyacrylic acid, which may be dissolved in an alcoholsuch as ethanol. The third polymeric material—a hydrophobic resin may bean acrylic thermoplastic such as for instance Setalux 17-1354. Suchmixtures may be deposited and treated in a similar fashion to thewater-based coatings and can be used for the automated or less automatedapplications. They are more applicable to the less automatedapplications where the plastic coating is produced in an industrialcoating environment, where solvent is recovered or incinerated asintegral to the coating procedure. The automated applications, whichwould be situated in a customer end-user environment, are moreproblematic as they involve having to deal with “VOC's” (volatileorganic compounds), which may provide health as well as environmentalhazards in these situations.

Whereas a wide range of lacquers may be employed for the final stage ofthe process, it has been found that water-soluble amino-plasts, togetherwith suitable acid catalysts, give excellent results. Examples of suchresins are Cymels 373 and 385 (partially methylated melamine resins),Cymels 1171, 1172 (glycoluril resins), Cymels 323, 328 and 327(methylated high imino melamine resins), UFR 60 and Dynomin UM-15(methylated urea resins). Preferably the catalyst is only liberated byheat so that the coating solution has optimum pot life. An example ofsuch a catalyst is Cycat 4045 (amine blocked para toluene sulfonicacid). Resulting coatings may be glossy, or a filler can be added toproduce a matt effect. Whilst the use of a water-based lacquer may nothave been anticipated as working on a water-based ink jet ink because ofthe danger of bleeding, not only was it found to work in the type ofsubstrate coatings described above, but it provides an optimal solutionwhere the customer would be doing the lacquering and where solvents orUV can be avoided and a least harmful water system chosen instead. Inaddition to amino-plasts which are cross-linkable, water-based emulsionshave also been found to be suitable. Examples of suitable emulsions areFlexobonds 325 and 381, Walpol 40-136, Synthamul 40412-03 (carboxylatedacrylic) and Arolon 880. After evaporation of the water, they formwater-proof films without the need to cross-link and such emulsions haveextremely long pot-lives. Also, the temperature at which water can beevaporated from the coating is lower than that needed to cross-linkamino-plasts.

In order to ensure that the final imaged material is sufficientlyresistant for applications, a number of tests were conducted.Rub/abrasion tests were done using an ATTC Crockmeter model CM5 andfollowing Test Method 8. Imaged material without an overcoat was testedwith dry rubbing and after 4 rubs generally showed very poor rubresistance with the image being almost entirely removed in some areasand the Crock squares showing high color transfer. Material aftercoating was tested for up to 100 rubs both dry and wet and with water,alcohol, petroleum spirit and methyl ethyl ketone.

The following examples illustrate the processes as described. Allformulations are given by weight.

EXAMPLE I

12% solution of polyvinyl alcohol in water. 2 35% solution ofpolyacrylic acid in water 5 Water 14.3 BYK 346 0.3 Walpol 5.0 Ethanol6.3 Kronos 2065 5.6This mixture was ball-milled for 2 hours to disperse the Titaniumdioxide and then wire rod coated on clear 175-micron polyester. It wasdried at 110° C. for 2 minutes to produce a film of 10 grams per squaremeter.

The sheet was imaged in an Epson Stylus C82 ink jet printer and thenheated for 4 minutes at 140° C. The following mixture was made up:

Water 3 Cymel 385 2.5 Superwetting Agent Q2-5211 0.1 Cycat 4040 0.1This was coated to a thickness of 4.6 grams per square meter anddried/cured in an oven at 140° C. for 4 minutes.

EXAMPLE II

Klucel E 1.26 Polyacrylic acid 6.79 BYK 346 1.88 Setalux 17-1745 13.7Kronos 2065 15.96 Ethanol 55.28 Ethyl Acetate 5.13This was ball milled for 2 hours and then coated and dried at 110° C.for 2 minutes onto white polyester to give a dry coating weight of 9.5grams per square meter. This material was imaged in an Epson Stylus C82ink jet printer and the sheet heated for 4 minutes at 140° C. to dry outthe material. The sheet was then coated with Flexobond 325 with a layerthat dried at 120° C. for 2 minutes to a dry weight of 7 grams persquare meter.

EXAMPLE III

Klucel E 1.5 Polyacrylic acid 7.93 BYK 346 2.27 Setalux 17-1354 15.9Ethanol 66.32 Ethyl Acetate 6.07

This was coated onto 175-micron transparent polyester and dried at 110°C. for 2 minutes to give a dry weight of 2.34 grams per square meter.The sheet was then imaged on an Epson Stylus C82 ink jet printer to givea laterally inverted image and then heated to 140° C. for 4 minutes todry out the inks. This was then laminated onto a pressure sensitivecoated white PVC (polyvinyl chloride). The image could be viewed throughthe polyester that provided a physical and chemical barrier to wear.

SOURCES OF RAW MATERIALS

-   Flexobond 325, Flexobond 381. Air Products and Chemicals, Allentown,    Pa., USA.-   Walpol 40-136, Synthamul 40412-03, Arolon 880. Reichold Inc.,    Research Triangle Park, N.C. USA.-   Setalux 17-1354, Setalux 17-1745. Akzo Nobel, Maastricht, The    Netherlands.-   Cymel 373, 385, 1171, 1172, 323, 328, 327. UFR 60, Dynomin UM-15,    Cycat 4045, Cycat 4040. Cytec Industries, Five Garret Mountain    Plaza, West Patterson, N.J., USA.-   BYK 346 BYK-Chemie GmbH, Postfach, 100245, Wesel, Germany.-   Kronos 2065 Kronos Inc. Huston, Tex., USA-   Klucel E—Hercules Inc. Wilmington. Del., USA.-   Super Wetting Agent.Q2-5211. Dow Corporation. Midland, Mich., USA.

1. A method of producing ink jet printed images with high resistance tophysical and chemical damage on plastic surfaces, comprising the stepsof: a. providing a plastic object; b. coating at least part of thesurface of the plastic object with an under-coating solution that doesnot contain a reactive species; c. evaporating off the volatile part ofthe under-coated solution, thus depositing a solid film on the surfaceof the plastic object; d. providing at least one ink-jet ink comprisinga colorant and aqueous carrier; e. jetting the ink by means of anink-jet system onto the surface of the under-coating on the plasticobject; f. warming the printed surface to drive part of the water in theink into the under-coating and to evaporate the remaining part of thewater; g. over-coating the dried surface with a water-based coating; andh. heating the over-coating to dry or cure it and to seal-in thepreviously deposited ink and under-coating.
 2. The method of claim 1,wherein the under-coating solution contains a mixture of hydrophilicpolymer or polymers together with a hydrophobic polymer; and whereinnone of the polymers in the undercoating react with other polymers inthe undercoating.
 3. The method of claim 1, wherein the ink-jet inkcolorant is a dye or a pigment.
 4. The method of claim 1, wherein theplastic object is a card or a bottle.
 5. The method of claim 1 where theplastic object is composed of a polyester or polyvinyl chloride.
 6. Themethod of claim 1, wherein said water-based coating for over-coatingcomprises water-soluble amino-plasts and acid catalysts.
 7. The methodof claim 1, wherein said water-based coating for over-coating compriseswater-based emulsions or a water-based emulsion plus alkali.
 8. Themethod of claim 1, performed as a single composite action, which isautomated to the extent that at no stage involves human handling.
 9. Themethod according to claim 8, wherein a plastic bottle is under-coated,imaged and over-coated.
 10. The method according to claim 1, wherein nochemical reaction occurs.
 11. The method according to claim 1, whereinthe under-coating is dried in sheet form for subsequent imaging andover-coating.
 12. The method according to claim 11, wherein a plasticcard is under-coated, imaged and over-coated.
 13. The method accordingto claim 1, wherein a plastic card is coated and imaged.
 14. The methodaccording to claim 1, wherein a plastic bottle is coated and imaged. 15.The method according to claim 14, wherein said ink-jet image comprises acolor image, additionally comprising a white under-layer, said colorimage viewable without influence of any contents of said bottle.
 16. Anobject imaged according to the method of claim 1, wherein the imagesurface is resistant to at least 100 strokes of the Crockmeter, dry orin contact with solvents, thus exhibiting high physical and chemicalresistance.